The GMT Ground-Layer AO Experiment at the Magellan Telescopes

摘要

It has recently been suggested that up to half of the wavefront variance can be removed from the total atmospheric distortion by correcting only the lowest seeing layer (Rigaut 2000, 2001). This Ground-Layer AO (GLAO) correction could provide improved image quality over a very wide field of view; however, no development work has been done on existing telescopes. The implications are profound for optical designs of future AO optimized telescopes (e.g. the ELTs) as accurately compensating for this ground-layer strongly favors an adaptive element conjugated to the median height of the ground-layer. The gains of GLAO are tantalizing but substantially unproven, and thus, the Giant Magellan Telescope (GMT) project has developed a multi-phased study with the goal of providing an on-sky demonstration of GLAO technology at the Magellan Telescopes. The first phase of this experiment is to measure the the height and boundary of the ground-layer through multiple, fixed wavefront sensors on very bright cluster fields over the full 24 arcminute Magellan field of view. With a typical wind speed of 9 m/s and a presumed secondary ground-layer conjugation error of 100 m, the equivalent decoherence time is approximately 0.04 seconds. Therefore, we have designed and constructed high resolution Shack-Hartmann sensors running at 100 frames per second with coarse, 0.5m sub-apertures. We present a technical description of the wavefront sensors and image analyzer, as well as current results from the first deployment of this instrument at Magellan. In addition, we discuss the implications for ground-layer modeling and describe the next phases of the GMT's GLAO experiment.